1. Field of the Invention
This invention relates to systems utilizing cards for storing data, particularly including set-top-boxes for use in pay-television systems.
2. Description of the Related Art
A smartcard is a card with an embedded microchip that can be loaded with data. Smartcards are often used in the field of pay-television where a card may be loaded with data including the viewing rights of the card holder. The card may then be inserted into a set-top-box to allow access to particular services. Smartcards also have extensive use in many other applications.
In many smartcard based systems, the detection of an inserted smartcard is used to trigger particular system processes. For example, for security reasons, insertion of a smartcard into a set-top-box may cause a system reset prior to reading data from the card. Detection of a smartcard is often performed by means of a contact signal that is generated when the smartcard makes electrical contact with a card reader. The state of a contact signal thus allows a smart card to be detected. However, smartcard interfaces are mechanical in nature and this can result in certain problems. For example, at the moment of insertion, the electrical contact between the card and the card reader may be temporarily unstable due to small vibrations and other mechanical perturbations of the card, sometimes referred to as bouncing. This causes noise to occur in the contact signal and rapid fluctuations between high and low signals. During this period, it is difficult to determine when stable electrical contact between the card and card reader has been made.
To overcome this problem, in some prior systems a long period of time is allowed to pass to ensure that card bouncing has ceased before the contact signal is analyzed. In other prior systems, a resistor and capacitor filter arrangement is provided to smooth the contact signal and remove noise over a long time period of time. However, in pay-television systems, it is desirable to perform a system reset in as short a time as possible after inserting a smartcard. If a long time delay is present between inserting a smartcard and performing a system reset, as occurs with the prior methods described above, the system may be vulnerable to hacking prior to the system reset.
One known technique for determining when a sufficiently noise-free contact signal is present is to periodically sample the contact signal. If successive sampled signals have the same value, then the signal is assumed to be stable. One problem associated with this technique is that brief periods of transient stability in the contact signal may result in multiple erroneous system resets.
We have appreciated that it is desirable to provide a means for determining when a stable signal has been established in as short a time as possible. We have also appreciated that it is desirable to eliminate erroneous resets.